1. Field of the Invention
The present invention relates generally to a method and apparatus for delivering materials to mixing or processing bins, and more particularly, to a method and apparatus for delivering a measured or metered amount of lightweight, friable (i.e., easily torn or otherwise damaged) mesh fibers including a rotatable intake hopper with an inner spiral guide, stationary flexible blades to facilitate movement of fibers on the upper and lower surfaces of the spiral guide, stationary blades for controlling the formation of bridges and clumps of the fibers, and a vacuum-based delivery portion at the outlet of the hopper that work in combination to minimize tearing or otherwise damaging the delivered fibers and to control clogging and binding in the hopper.
2. Description of the Related Art
It is a growing trend in the construction and structural prefabrication industries to reinforce concrete, and similar materials, by adding a known amount of synthetic mesh fibers during the initial mixing or production of the concrete. The addition of specific quantities of these mesh fibers has proven useful for inhibiting shrinkage cracking, increasing impact capacity, reducing permeability, and providing other improvements in the physical characteristics of the end product fabricated from the reinforced concrete. Typically, it is desirable to add somewhere between a 1/4 pound and 10 pounds of the mesh fibers per cubic yard of concrete. Due to the lightness of the mesh fibers, this range of weights of mesh fibers represents a relatively large volume of the mesh fibers which must be accurately measured and transported to achieve the desired ratio of mesh fibers to other concrete components. Unfortunately, prior to the present invention, the delivery of a specific amount of undamaged mesh fibers has caused the construction and structural prefabrication industries a number of serious problems.
One problem that these industries face is delivering the mesh fibers to the concrete mixing tank without damaging the mesh fibers such that the mesh fibers are not effective as a reinforcing additive. In this regard, FIGS. 1A and 1B illustrate, respectively, a commonly used synthetic mesh fiber 2, as disclosed in U.S. Pat. No. 5,456,752 of Hogan, in a compact state and in an expanded state. The expanded state is desired during mixing to enhance mixing with cement, line, aggregate, and other materials used in forming the reinforced concrete because these materials fill the spaces in the mesh fiber 2 and better bond with the mesh fiber 2. In the expanded state of the mesh fiber 2, the main fibrils 4 and side fibrils or members 6, 8 and the spaces are clearly visible, and for the mesh fibers to be effective as an additive, it is important that the main and side fibrils 4, 6, and 8 remain intact and that the side fibrils 6, 8 remain attached to the main fibrils 4. The fibers are typically fabricated from lightweight materials such as polypropylene, polyethylene, polyester, polyvinyl chloride, and polyamides with very fine (e.g., 360 to 2600 deniers and a thickness of 0.0001 to 0.01 inches) main and side fibrils 4, 6, and 8 with a length of 0.4 to 1.5 inches and a width of 0.05 to 0.3 inches. Consequently, delivering the mesh fibers without damage is difficult because the mesh fibers are relatively fragile and friable.
Another problem these industries face is how to provide a metered or measured quantity of the mesh fibers. The mesh fibers are small in size and very lightweight. The use of a simple gravity feed hopper is ineffective because the weight of the fibers themselves is not enough to overcome friction that develops between the fibers and between the fibers and the sides of the hopper, and the hopper typically becomes clogged with fiber. U.S. Pat. No. 5,775,852 of Boutte et al. discloses a system that utilizes load cells to measure quantities of dry bulk powder removed from containers and then delivered with a vacuum pump to a mixing bin, but Boutte et al. appears to rely on gravity acting on the dry powder material along with a vacuum force to remove powder from the material containers. However, this system would be less effective for a lightweight material such as the mesh fibers for which gravity forces are not as great as friction forces between the fibers and would most likely result in a very slow dispensing process and/or clogging in the discharge hopper. In practice, the tendency of the mesh fibers to cling to adjacent fibers is large enough that without agitation of the fibers the fibers tend to bind together forming clumps that bind to adjacent surfaces creating binding and bridges or levels of fibers that are strong enough to resist gravity forces.
U.S. Pat. No. 5,407,139 of Mleczewski and U.S. Pat. No. 5,829,649 of Horton each teach systems for delivering insulation material. However, these systems teach the use of screens, augers, shafts, and the like for forcing the material downward in a bin or hopper and also for agitating and separating the material. The systems disclosed by Mleczewski and Horton would be ineffective for mesh fibers 2, shown in FIGS. 1A and 1B, because the fibers 2 are so fine and lightweight that internally rotating augers and paddles merely churn the mesh fibers and become quickly clogged with the mesh fibers 2. The fibers tend to bind together into clumps and strands that wrap around agitators, such as shafts, augers, and paddles, that are rotated within the fibers in a hopper. Additionally, a large percentage of the mesh fibers 2 that are eventually forced through the feed hopper are typically damaged as the thin main and/or side fibrils 4, 6, 8 are broken. Another shortcoming with the Mleczewski and Horton systems for use in delivering mesh fibers 2 is the teaching of a blower to forcefully blow or push material out of the discharge of the system or hopper. This delivery method is not useful for delivery of mesh fibers 2 because of the friable or separable nature of each mesh fiber 2. A mesh fiber 2 generally would expand as shown in FIG. 1A when exposed to blowing forces and, depending on the length of the delivery pipe or tube, the fibrils 4, 6, 8 would become torn or otherwise damaged.
Due the limitations with existing mechanical delivery systems, these industries have had to rely on manual methods of adding the mesh fibers to concrete mixing tanks or other mixing devices. More specifically, delivery of a "measured" amount of the mesh fibers to a concrete mixing tank is accomplished by having several workers manually throw a predetermined number of bags of the mesh fibers into an opening in the tank at a concrete plant or into the mixing tank of a ready mix truck. The bags are fabricated of unique material that degrades during mixing but remnants of the bag often remain in the concrete, especially if the bags are added too late in the mixing process. This method of adding the material is labor intensive which increases costs and worker safety concerns and is unreliable for achieving the type of mixing needed to obtain the fill benefits of adding the mesh fiber material to the concrete.
Another concern for the addition of the mesh fibers to concrete is that the delivery or addition of the fibers must be done in a relatively short time because the fibers are added only when the concrete mix is charging, i.e., when additives are being loaded. This provides only a short window of opportunity for delivering the fibers to the concrete mixing or processing bin.
Consequently, there remains a need in the concrete and structural prefabrication industries for a reliable, safe, cost-effective, and time-efficient (i.e., within charging window) method and apparatus for delivering a known quantity of mesh fibers to a concrete mixing tank or other mixing device. Further, it is desirable that such delivery method and apparatus be able to convey the mesh fibers with little or no damage to the mesh fibers such that the bonding and reinforcing characteristics of the mesh fibers are retained.